Basal Metabolic Rate

The cumulative increase in BMR can comprise a large part of the total energy costs of pregnancy. Although 150 MJ is a good estimate of the average energy cost of maintenance for a well-nourished woman, there is a very wide range. This has an important influence on the extra daily requirements for individual women. Studies in which BMR has been measured every 6 weeks from prepregnancy to 36 weeks of pregnancy have shown very marked differences. In some women, there is the expected response to pregnancy—an immediate and progressive increase in BMR. In other women, BMR actually decreases or increases only slightly in the early stages of pregnancy and does not increase substantially until late gestation. This offsets the later increase in BMR such that there is actually a slight net saving of energy over the entire gestation period in some of these 'energy-sparing' women. The total net cost of maintenance, estimated as the cumulative area under the curve represented by the rise in a mother's BMR above the prepregnancy baseline metabolic rate, is negative or only very small. Data indicate that this between-subject variability is found in women from both well-nourished and marginally-nourished populations. However, 'energy-sparing' and 'energy-profligate' responses dominate in marginally and well-nourished women, respectively. There is a more than 5-fold range between the most energy-profligate and the most energy-sparing women.

In addition to the wide variability in changes in BMR between individual women, there are also wide variations between different populations. Well-nourished affluent women from developed countries tend to show an energy-profligate increase in BMR. In marginally nourished thinner women from developing countries the increase in BMR is delayed and/or preceded by a decline in early pregnancy. The total maintenance costs of pregnancy in these studies range from +210 MJ (+50 000kcal) to -45 MJ ( — 11 000kcal).

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